In the recent literature, there has been much interest in electrochemical processes which cause semiconductors, such as silicon, to become porous. Various articles have appeared in Applied Physics Letters and other publications relating to such devices. See for example, an article by V. Lehman and U. Gosele, Applied Physics Letters, Volume 58, Page 856 (1991). See also an article in Volume 57 of Applied Physics Letters, by L. T. Canham, Page 1046 (1990). The prior art was cognizant of the fact that in certain instances, porous silicon exhibits unique properties which are superior to those of bulk silicon. For example, high efficiency luminescence has been observed in porous silicon above the 1.1 eV band-gap of bulk material, which suggests that optical devices can be fabricated based on the use of porous silicon. Control of the pore size on the nanometer scale can allow porous materials to be used as filters in solid state chemical sensors. Furthermore, because of the large surface area inherent in porous structures, the porous semiconductors exhibit oxidation rates which are many orders of magnitude above bulk crystals. This effect can be used for selective etching of the semiconductor, since oxides are easily removed from the semiconductor surface. It also can be used to dielectrically isolate devices fabricated in the semiconductor wafer.
In any event, there are several theories for the formation mechanisms of pores in silicon. A good reference is the article by R. L. Smith and S. D. Collins appearing in the Journal of Applied Physics, Volume 8, R1 (1992). Studies suggest that the depletion regions of pores overlap, causing a carrier depletion in the interpore region, and thus the current is confined to the pore tips. In an article that appeared in the Journal of the Electrochemical Society, Volume 138, Page 3750 (1991) by X. G. Zhang, there was indicated that pore propagation is attributed to a higher electric field at the pore tips which causes dissolution to occur more rapidly through the intermediate step of silicon dioxide formation, while along the pore walls dissolution occurs through the slower process of direct dissolution. The pore wall thickness stabilizes when the depletion regions of pores overlap. In the above-cited article of Lehman et al., it is claimed that the current is confined to the pore tips due to the wider band-gap in the quantum size porous material, which causes carriers to be confined to the lower band-gap bulk material. Pore initiation may occur at asymmetrical surface sites, such as defect or impurities, or through chemical nonuniformities which occur in the initial stages of anodization of the surface.
Recent demonstrations of room temperature visible luminescence from porous silicon have generated much interest in using the material for optoelectronics. There is of course much conjecture about the mechanisms which provide the visible luminescence. Certain people claim that luminescence is caused by quantum structures (wires or dots) in the porous silicon. Presumably, these quantum structures would allow a relaxation of the momentum selection rules by confining the charges spatially, thus allowing direct band-gap transitions. Additionally, the charge confinement would increase the effective band-gap, thereby pushing it into the visible region.
Others such as C. Tsai, K. H. Li and D. S. Kinosky, et al., in an article in Applied Physics Letters, Volume 60, Page 1770 (1992) have shown that surface chemistry, specifically hydrogen termination, play an important role in the luminescence. This suggests that luminescence in porous silicon may have similar mechanisms as a-Si, which exhibits band gap widening into the visible region when hydride species are formed on the surface. It has yet to be conclusively determined whether the hydrogen termination serves only to passivate the surface and the luminescence is caused by a purely chemical effect. Nevertheless, it is very clear that microcrystals of &lt;5 nm dimension can theoretically exhibit band gap widening and above band-gap luminescence.
There has been interest in SiC as a semiconductor material since the 1950's. Its wide band-gap, high thermal conductivity, high breakdown electric field and high melting point make SiC an excellent material for high temperature and high power applications. SiC also exhibits interesting optical properties, such as deep UV absorption, visible transparency and blue photo- and electro-luminescence. However, good quality crystals were unavailable, causing the early research efforts to stagnate. Recent developments in single crystal epilayer and boule growth have generated new interest in SiC. This has resulted in the development of SiC blue LED's, UV photodiodes and high temperature electronic components. However, due to its indirect band-gap, the efficiency of SiC optoelectronic devices is limited. Thus, research is underway to develop other wide band-gap semiconductors, such as SiC.sub.x A1N.sub.y and III-V nitrides for optical applications. However, the crystal growth technology for these materials is still very underdeveloped. Therefore, porous SiC could be very useful, since it has potentially superior optical properties than SiC, and may benefit from the relatively mature growth and processing technology that SiC has to offer. Furthermore, SiC is very difficult to etch because of its chemical inertness. Therefore, porous SiC could also be used to pattern this material for electronic device fabrication.
There have been several reports on the electrochemical dissolution of SiC. Recently, Shor, et al. used laser assisted electrochemical etching to rapidly etch high relief structures in SiC. See an article by J. S. Shor et al., in Journal of Electrochemical Society, Volume 139, Page 143, May 22, 1992. M. M. Carrabba et al., an article in Electrochemical Society Extended Abstracts, Volume 89-2, Page 727 (1989), reported etching diffraction gratings in n-type .beta.-and .alpha.-SiC at anodic potentials with a uniform light source. The fundamental electrochemical studies indicate that the presence of HF in aqueous solutions is important to etch SiC electrochemically. Glerria and Memming in Volume 65 of the Journal of Electronal Chem., on Page 163 (1975) reported that .alpha.-SiC dissolves in aqueous H.sub.2 SO.sub.4 solutions at anodic potentials through the formation of a passivating layer, which was suggested to be SiO.sub.2, since it dissolved in HF.
The present invention relates to the formation of porous SiC, a new material. It is indicated that porous SiC material itself, as well as a process to fabricate the porous SiC is provided. Porous SiC can be employed for UV light sources such as LED's and diode-lasers. Porous SiC can be utilized as a filter in chemical processes and can be used to provide heterojunction devices using the porous SiC/bulk SiC interface. As will be described, the methods employ a selective etching of bulk SiC by forming a porous layer on the surface, oxidizing it and stripping it in hydrofluoric (HF) acid. One can also provide dielectric isolation of SiC devices on a wafer.